Some grinding machines have an automatic operation mode in which a workpiece is automatically machined by automatically moving the position of a grinding wheel relative to the workpiece in accordance with a program stored in a numerical control apparatus or the like and a manual operation mode in which the workpiece is machined by an operator's manual operation in which the operator operates a manual rotating handle to move the position of the grinding wheel relative to the workpiece.
In conventional hydraulic grinding machines with the manual operation mode, the position of the grinding wheel relative to the workpiece is moved by using the manual rotating handle to apply pressure directly to hydraulic oil for oil pressure that al lows movement of the position of the grinding wheel relative to the workpiece. Thus, when the grinding wheel contacts the workpiece, the grinding wheel (or the workpiece) does not move even if is applied to the hydraulic oil. Thus, a rotational torque that is a torque needed to rotate the manual rotating handle is automatically increased, so that the operator can easily feel the contact between the grinding wheel and the workpiece. In the conventional grinding machines, the operator can recognize subtle (delicate) contact based on the increased rotational torque of the manual rotating handle, and can start to finely (delicately) operate the manual rotating handle after the contact to achieve accurate machining.
Patent Document 1 describes a CNC grinding machine including a table feeding manual handle (corresponding to the manual rotating handle) and a wheel spindle stock-feeding manual handle (corresponding to the manual rotating handle) and having a manual operation mode. In the manual operation mode, each manual handle causes a pulse generator to generate a pulse corresponding to the amount of rotation, and the pulse is input to a control unit that outputs an amount of control corresponding to the pulse to a servo motor to move the position of the table or the grinding wheel.